In the past 5 years there has been a significant increase in the demand of uranium for base load power generation. New mines have been developed to meet this demand. Conventional processing of uranium ores requires leaching the uranium bearing minerals in sulphuric acid or sodium carbonate under oxidising conditions to liberate the uranium as a soluble uranyl sulphate or uranyl carbonate complex. Uranium deposits situated in arid geographical regions require large quantities of desalinated water in the case where ion exchange is selected as the process technology. Large desalination plants have been built and others planned for a number of proposed uranium projects.
Leaching of uranium minerals can be performed by various techniques. If the ore body is below the water table in a confined aquifer then in-situ leaching is possible. In-situ leaching (ISL) is where dilute sulphuric acid is injected into the ore body through a series of injection wells. The acid reacts with the uranium bearing minerals and leaches them into solution. A series of recovery bores then extract the weak acidic solution containing the uranyl sulphate from the aquifer (IAEA, 2001). The pregnant leach solution (PLS) from the recovery bores typically contains 50-200 ppm uranium in solution. This uranium concentration is low compared to conventional agitated tank leaching. In the case of ISL, ion exchange (IX) is normally used to recover and concentrate the uranium from solution. Solvent extraction (SX) can be used for recovery of uranium from ISL liquors, however due to the low uranium concentration in solution it is normally proven to be uneconomic. Once the uranium has been recovered the barren PLS is re-acidified before being re-injected into the bore field. In arid regions the ground water is normally very saline and uranium recovery using resin is not possible.
Heap leaching is also employed to extract uranium from low grade ores. The ore is crushed and stacked into heaps and sulphuric acid or sodium carbonate irrigated on to the heap. The leach solution percolates through the heap and the pregnant leach solution collected in a pond (IAEA, 1993). Again the solution typically contains very low concentrations of uranium (50-200 ppm uranium) and IX is used to recover uranium before the solution is recycled to the heap leach circuit. In countries such as Namibia there are vast amounts of low grade uranium ore amenable to heap leaching. Large desalination plants have been built and more proposed to exploit these deposits.
Hard rock uranium ore bodies or those ore bodies above the water table are processed by conventional open pit or underground mining techniques. These ores are normally crushed and then milled to the correct liberation particle size to ensure maximum extraction is achieved during uranium leaching. Either sulphuric acid or sodium carbonate is normally employed as the leaching reagent. Elevated temperature and pressure can be used to reduce leach residence time or to improve overall uranium leach extraction. Once the ore has been leached the pregnant leach solution is normally recovered through a counter current decantation (CCD) circuit to wash the leach residue and recover the soluble uranium. A large amount of wash water is normally required to ensure a high wash efficiency is achieved. Uranium in solution is then recovered by solvent extraction, ion exchange or precipitation. If high chloride levels are present in solution, which originated from the ore, addition of a chloride oxidant or water used in the process, then ion exchange cannot be used. At elevated chloride levels only solvent extraction or direct precipitation has been available as a technically feasible option. Resin-in-pulp (RIP) can also be employed where the resin is contacted directly with the leach discharge pulp and uranium recovered from the pulp (Gupta & Singh, 2003). This avoids the requirement of a CCD wash circuit. Again RIP is only applicable when chloride concentrations are maintained below 3 g/L in the leach pulp.
Strong base anionic (SBA) resins have been widely adopted as the conventional approach for extraction of uranium from an acidic or alkaline PLS or pulp. An SBA resin has a quaternary amine functional group and chemically adsorbs uranyl sulphate or uranyl carbonate anionic complexes by electrostatic forces. Chloride is also a strong anionic ion and when present in high concentrations it competes with the uranyl complexes. As the chloride concentration increases, loading of uranium onto SBA resins decreases. All SBA resins preferentially load chloride and once the chloride concentration typically exceeds 3.5 g/L then this starts to significantly impact the loading capacity of the ion exchange resin. For example, a strong base anionic (SBA) resin (Dowex 21K) has an equilibrium loading of 20 g U3O8/L wet settled resin (wsr) loading when contacted with a solution containing 150 ppm U3O8 and 3.5 g/L chloride at pH 1.8. This loading decreases to 7 g U3O8/L (wsr) at 7.5 g/L chloride and 4.3 g U3O8/L (wsr) at 8.3 g/L chloride (La Brooy, 2009). Weak base anionic (WBA) resins containing secondary and/or tertiary amine groups are used when high sulphuric acid concentrations are present after leaching. WBA resins are also susceptible to low loadings when high chloride concentrations are present in solution. Improved methods and/or resins are required.